AN-Short character exploration for both Light and L.
Light knows that people don't like extremes. Somebody that's too much of something seems to unnerve people; because nobody should be too far from average, really. Extreme people are dangerous, because whatever is extreme about them gives them some advantage and it is intimidating. Or possibly it reminds them of something about themselves, something lacking or inevitable. Average is regular, it is understandable, it is trustworthy.
Light is anything but average. And Light also knows that it's not always good to have people not like you. If people like you, they will do things for you. He learnt that young. The art of manipulation is not a difficult one to learn and it can be used well to disarm. This boy is so smart, good at everything he does. He might be insufferable but he's not because he cares about you, he wants the world to be good for you and he wants you to learn and see things his way and you will. He's so lovely, so of course he couldn't be anything but right, could he?
Light wraps his cold, solid core of genius up in charm and sweetness and friendliness.
L keeps his exposed. Either because he doesn't know that other people find him uncomfortable or he doesn't care. And why would he care, really, since he's got the world's major forces at his fingertips but strip him of his fame and his influence and leave him simply as the creature he is and then compare him.
Light is fully aware that taken on his own, people find L awkward, unnerving and strange and not just because he is but also because he's too clever. He knows too much and that almost makes him more vulnerable than if he knew too little. Oh, he can manipulate because he is clever but there's no charm to calm the unnerved, there's no sparkling smile to disarm the intimidated and there's no honeyed words to cheer up the inadequate.
L is hard, naked genius. That is frightening. People turn against those who frighten them. Beneath his charismatic exterior, Light knows this is something he could use.